The present invention relates to a stepping motor used mainly for a data processing apparatus, and, more particularly, the invention relates to the structure of a rotor for the stepping motor and material used for forming the rotor.
In recent years, under the circumstance that the performance of a data processing apparatus is improved, the improvement of the performance of a stepping motor used for the apparatus is also required.
In regard to the improvement of the performance of the motor, the decrease of the vibration and the noise of the motor, and the improvement of the efficiency of the motor are mostly required. At the same time, the improvement of the productivity of the motor having high performance is also required.
In general, a rotor for the stepping motor has the structure that a cylindrical magnet and a shaft are united with a molded-member, and, in a conventional motor, the molded-member has been formed of thermoplastic resin added fillers such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT).
In the following, the rotor for the conventional motor is described on reference to FIG. 7 and FIG. 8 which are cross sectional views showing the rotors having respectively different structure.
In FIG. 7, a cylindrical magnet 119 having a plurality of magnetized poles along the circumference thereof and a shaft 118 are united in the state that the shaft 118 and the cylindrical magnet 119 are coaxially disposed, with a molded-member 123 formed by injection molding. The injection molding is performed in such a manner that the thermoplastic resin is injected into a space formed between the cylindrical magnet 119 and the shaft 118. The shaft 118 has a knurled portion 120 on the surface thereof for increasing the strength of uniting the shaft 118 and the molded-member 123.
In FIG. 8, a first cylindrical magnet 125 and a second cylindrical magnet 126, each of which has a plurality of magnetized poles along the circumference thereof, an annular spacer 129 which is disposed between the first magnet 125 and the second magnet 126, and a shaft 124 are united in the state that all of these components of the rotor are coaxially disposed, with a molded-member 131 formed with the thermoplastic resin in a manner similar to the above description on FIG. 7. The shaft 124 has a knurled portion 130 on the surface thereof for increasing the strength of uniting the shaft 124. and the molded-member 131.
However, in the conventional rotor in which the molded-member (123 or 131) is formed with the thermoplastic resin, the sufficient dimensional preciseness of the rotor is hardly obtained since the molding shrinkage of the thermoplastic resin in forming the molded-member and the shrinkage of the molded-member after the molding are large.
For improving the dimensional preciseness of the rotor, a proposal is disclosed in Japanese Patent Publication No. 2640413. According to the publication, it is described that the vibration and the noise of the motor can be reduced by forming the molded-member with a liquid crystal polymer in place of PBT or metal which has been conventionally used, since the vibration damping characteristic of a liquid crystal polymer is superior than that of the thermoplastic resin or metal. Also, it is described in the publication that the vibration of the motor can be reduced because the inertia of the rotor is reduced by using a liquid crystal polymer in place of metal. Also, it is described in the publication that the dimensional preciseness of the rotor can be maintained after the molding process of the molded-member by using a liquid crystal polymer, and it is also described that the efficiency of producing the rotor can be improved because the fluidity of a liquid crystal polymer is superior.
It is known that a liquid crystal polymer is superior than the thermoplastic resin in the characteristics described above, such that the improvement from the conventional motor can be expected by using a liquid crystal polymer.
However, a liquid crystal polymer has a characteristic that the molding shrinkage largely changes depending on the flowing direction thereof in a molding process. That is, the molding shrinkage ratio of a liquid crystal polymer is 0.15% in the flowing direction, and 0.45% in the right angle direction of the flowing direction. On the other hand, the molding shrinkage ratio of thermoplastic resin such as PBT is as large as approximately 0.3%-1% though the directional dependency (i.e., the change of molding shrinkage due to the flowing direction) is not large. As the above figures show, although the molding shrinkage of a liquid crystal polymer is small in the flowing direction comparing with that of the thermoplastic resin, the molding shrinkage in the right angle direction of the flowing direction is considerably large. Therefore, in using a liquid crystal polymer for the molded-member which unites the components of the rotor, it is necessary to pay attention on the shape of a mold for forming the molded-member, and on the gating of the mold, also it is necessary to pay attention on the method for preventing the decrease of mechanical strength at the weld line of the molded-member.
Also, although it is described in the publication that the fluidity of a liquid crystal polymer is superior than that of the thermoplastic resin, the pressure required for forming the molded-member by the injection molding of a liquid crystal polymer is as high as 300 kg/cm2, which result in the occurrence of damage on the magnet, which is rather fragile, in the production process of the rotor. Due to the above disadvantage of a liquid crystal polymer, a rotor having a cylindrical magnet whose axial dimension is relatively large against the inside diameter of the cylindrical magnet is hardly produced by using a liquid crystal polymer.
Also, in the case where a rotor having large inertia is required, neither liquid crystal polymer nor the thermoplastic resin is hardly used due to the large molding shrinkage. That is, in the case where the material such as metal, whose specific gravity is large, is used for the spacer 129 for increasing the inertia of the rotor, the vibration of the motor becomes large since sufficient shaft balance of the rotor is hardly obtained by using a liquid crystal polymer or the thermoplastic resin due to the molding shrinkage characteristic described above.
The object of the present invention is to address the problems in the conventional stepping motor, and to provide a stepping motor having a rotor which has a good shaft balance for reducing the runout of the circumference of the magnet of the rotor, by which the vibration and the noise of the motor can be reduced and the efficiency of the motor can be improved. A further object of the present invention is to provide a low cost and high performance motor having a rotor which has relatively long axial dimension against the diameter of the rotor for improving the response characteristic of the motor by reducing the inertia of the rotor. A still further object of the present invention is to provide a low cost and high performance motor having a rotor whose inertia is large in the case where such a rotor is required.
For realizing the above object, the motor of the present invention comprises the following elements.
(a) a rotor including
a cylindrical magnet having a plurality of poles magnetized in a circumference direction; and
a shaft coaxially incorporated into the magnet with a molded member made of thermosetting resin,
(b) a stator facing to the rotor via an air space.
In the above structure, since the molding shrinkage of the thermosetting resin contained in the mixture is small, the dimensional preciseness of the rotor can be improved, also, the process for annealing the molded-member can be exempted. Also, since the mixture containing thermosetting resin has a superior fluidity characteristic, a motor having a long and narrow rotor (i.e., a rotor whose relative axial dimension against the inside diameter of the cylindrical magnet of the rotor is more than 0.3) can be produced efficiently, such that a motor having superior response characteristic and less vibration can be realized.
Also, for realizing the above object, the motor of the present invention has the following elements.
(a) a rotor including:
two cylindrical magnets having a plurality of poles magnetized in a circumference direction and disposed coaxially with each other;
a spacer having a section contacting with inner walls of the two magnets and a section separating the two magnets;
a shaft incorporated with the magnets and the spacer coaxially with a molded member formed of thermosetting resin; and
(b) a stator facing to the rotor via an air space.
In the above structure also, since the thermal shrinkage of the mixture containing thermosetting resin is smaller than that of the thermoplastic resin and that of a liquid crystal polymer, dimensional preciseness of the rotor can be improved, such that the shaft balance of the rotor is improved. As a result, in the case where a motor comprising a rotor having large inertia is required, such a motor can be obtained by using a spacer whose specific gravity is larger than that of the molded-member, in which the vibration of the rotor is reduced to a sufficiently low level.